yann 8/2/2018 9:41 AM
VOX AC15 burning resistors [SOLVED]
Dear MEF members,

My name is Yann and I leave in France, this is my first post on this amazing forum.
I am a frequent visitor of this forum since about 1 year and I have to say it helped me a lot.

So I decided to post a thread about a repair I made on a VOX AC15. This one was a bit tricky so I hope this will help someone in the future and avoid some headache

This amp came to me after the customer faced a sudden buzz followed by smoke and a blown HT fuse.
After some checks on the circuit, I have noticed two 220 ohms burnt resistors (R90 & R91, schematic attached) in addition to the blown HT fuse :

[ATTACH=CONFIG]49946[/ATTACH]

After checking heater circuit and all tubes, absolutely nothing was wrong.
So I have decided to change the fuse, and put the amp on a variac and slowly start the beast.
The two resistors became very hot again, but I could see all filaments properly lighting.
After unpluging different secondaries of the power transformer, I have figured out that when the main secondary is unplugged the resistors are not longer cooking.

So I decided to plug again the main secondary and measure voltage on resistors, and noticed an abnormal high voltage on these.

For me it sounded like a partial short somewhere and I finally figured out after unsoldering C62, it was leaking on the pcb, causing the coat to dissolve and a partial short to happen, as the heater supply track was going in between pins of C62...

[ATTACH=CONFIG]49948[/ATTACH]

So to fix this, I have cut the track that was running under C62 and remove the remaining copper, made a strap connection for heater supply getting around C62 with a proper insulated wire and replaced C62 (sorry I don't have a picture for the fix...).

Hopefully this might help someone in the future.